In recent years, there has been an increasing need to drill wells for extracting petroleum, gas, water, hot water, hot springs, or the like from the earth or for surveying water quality (collectively called “wells” hereafter) in order to secure energy resources or protect the environment. In order to drill a well such as an oil well, for example, with an apparatus for digging a well, i.e. a well-digging apparatus, drilling is generally performed up to a prescribed depth from the earth's surface, and a steel pipe called a casing is laid therein so as to prevent the collapse of the wall. The well is dug further underground from the end of the casing to form a deeper well, and a new casing is laid through the inside of the casing laid previously. The diameter of the casing is adjusted as necessary, and this operation is repeated until an oil well pipe reaching an oil stratum is ultimately reached. Depending on the method of drilling, a casing is sometimes not used.
In the drilling of a well, a bit attached to a drill tip crushes the rock of the subterranean formation and advances through the well while rotating, and the crushed rock is carried out to the earth's surface. At the time of well drilling, a slurry-like dispersion for drilling (drilling fluid) obtained by dispersing a granular material such as bentonite, mica, slaked lime, carboxymethyl cellulose, or a silicone resin in a liquid carrier such as water or an organic solvent is used for the purpose of reducing friction between the drill and the well wall, cooling the bit, carrying out crushed rock or the like, preventing the lost circulation during the drilling operation, or preventing the collapse of the well wall formed by boring (Patent Documents 1 and 2). The drilling fluids including a drilling-mud, a completion fluid, and so on that are used are obtained by dispersing the granular material described above in a liquid carrier selected from water or an organic solvent such as a diol or triol such as ethylene glycol, propylene glycol, glycerol, or trimethylene glycol; a glycerol ester such as glyceryl triacetate (triacetin), glyceryl tripropionate (tripropionin), or glyceryl tributyrate (tributyrin); or a polyglycol such as polyethylene glycol together with additives such as a lost circulation material, a specific gravity control agent, a dispersant, a surfactant, a viscosity adjusting agent, or a thickening agent. In order to avoid obstructing the drilling operation, the granular material used in the drilling fluid must have fluidity, heat resistance, chemical stability, mechanical characteristics, and other properties, and it also needs to be possible to rapidly discharge and safely dispose of the drilling fluid without the mud cake layer being left behind upon the completion of the drilling operation. There is therefore a demand for a granular material or drilling fluid that satisfies these requirements.
On the other hand, in recent years, improvements in production technology have brought attention to drilling for unconventional resources so as to overcome the conventional peak oil theory, and techniques such as horizontal wells and hydraulic fracturing have been introduced. For example, hydraulic fracturing (fracturing) is known as a well stimulation method which improves production capacity or durability by creating cracks (also called fractures or bore holes) in the reservoir by applying a high pressure to the inside of the well and filling the cracks with a support material (proppant) such as sand to prevent the closure of the cracks, thereby forming channels (oil/gas pathways) with high permeability in the reservoir. Cracks are formed by injecting a high-viscosity fluid through the inside of the well from above ground. In order to increase the effect of fracturing against the high temperatures and high pressures in the ground, the selection of an injection fluid or a support material (proppant) for maintaining the cracks is extremely important. Sand is typically used as a support material, but it is necessary for the support material to have a spherical shape and uniform particle size in order to have strength to sufficiently withstand the closure pressure of crack and to keep the permeability of these portions high. Various types of water-based, oil-based, and emulsion-based injection fluid are used as the injection fluid. The injection fluid must have a degree of viscosity capable of carrying the proppant as well as good proppant dispersibility or dispersion stability, and there is a demand for the ease of after-treatment and a small environmental burden, so various additives such as gelling agents, scale preventing agents, acids for dissolving rock or the like, and friction reducing agents are used. For example, a composition comprising approximately from 90 to 95 mass % of water, approximately from 5 to 9 mass % of 20/40-mesh sand (proppant), and approximately from 0.5 to 1 mass % of additives may be used as the fluid composition for performing fracturing.
In the completed well, the product fluid such as petroleum is discharged to the earth's surface through the well of cased hole or open hole while being separated from gravel, sand, and the like. In the well production process, in addition to the use of the drilling fluid described above, cementing or plug (plugging) treatment may be performed from when drilling is begun until the completion stage for various purposes such as to protect the casing or to separate fluids from other layers by means such as blocking fractures or cracks, so that the fluids do not flow into the reservoir, for example. In addition, the repair of the well is also sometimes necessary due to changes over time. Furthermore, test drilling may be performed for the purpose of testing or inspection prior to well drilling. In order to implement these treatments, various well treatment fluids are used, and there has been a need to smoothen the recovery or reuse of the components of the well treatment fluids, to reduce the environmental burden thereof, or the like.
The idea of blending a degradable material into the well treatment fluids is known from the perspectives of the ease of the after-treatment of the well treatment fluids or the reduction of the environmental burden thereof. For example, the use of degradable resin particles in a fracturing fluid is disclosed in Patent Document 3, and it is also disclosed that the particles may contain fibers. In addition, it is disclosed in Patent Document 4 that a slurry containing a degradable material is injected as a temporary plug to be used temporarily at the time of well drilling, and fibers are described as the degradable material.
However, in these prior art documents, many resin materials are listed as degradable materials, an extremely large number of types of shapes and sizes are disclosed for the particles or fibers to be formed from the degradable materials. For example, in Patent Document 3, spheres, rods, plates, ribbons, fibers, and the like are listed as shapes of solid particles made of the degradable material. Resin fibers are also listed as fibers in addition to glass, ceramics, carbon, metals, and alloys. In Patent Document 4, shapes such as powders, particles, chips, fibers, beads, ribbons, plates, films, rods, strips, spheroids, pellets, tablets, and capsules are listed as shapes of the degradable material. Filaments (long fibers) and fibers with a length of 2 to 25 mm are also disclosed as fibers. That is, it is not clear what should be selected as an optimal degradable material.
On the other hand, since aliphatic polyester resins such as polyglycolic acid resins (sometimes called “PGA” hereafter) or polylactic acid resins (sometimes called “PLA” hereafter) are degraded by microorganisms or enzymes existing in the natural world such as in soil or in oceans (PGA or PLA forms acidic substances such as glycolic acids or lactic acids by hydrolysis, and these acidic substances are degraded into water and carbon dioxide by microorganisms or enzymes), attention has been focused on these resins as biodegradable polymer materials with a small burden on the environment. Since these biodegradable aliphatic polyester resins have biodegradable absorbent properties, they are also used as polymer materials for medical purposes such as surgical sutures or artificial skin.
Known biodegradable aliphatic polyester resins include polylactic acids consisting of lactic acid repeating units (PLA; in particular, PLLA consisting of repeating units of L-lactic acid, PDLLA consisting of repeating units of DL-lactic acid, and the like are widely known); PGA consisting of glycolic acid repeating units; lactone polyester resins such as poly-ε-caprolactone (sometimes called “PCL” hereafter); polyhydroxybutyrate polyester resins such as polyethylene succinate and polybutylene succinate (sometimes called “PBS” hereafter); and copolymers thereof such as copolymers consisting of glycolic acid repeating units and repeating lactic acid repeating units (sometimes called “PGLA” hereafter), for example.
Of these biodegradable aliphatic polyester resins, PGA has not only high biodegradability and hydrolyzability when an alkali solvent or the like, for example, is used, but also excellent mechanical characteristics such as heat resistance and tensile strength and, in particular, excellent gas barrier properties when used as a film or a sheet. Therefore, PGA is expected to be used as agricultural materials, various packaging (container) materials, or polymer materials for medical use, and applications have been expanded by using PGA alone or combining PGA with other resin materials or the like. PLA and PGA have also been described as examples of degradable materials in Patent Documents 3 and 4 described above.
In step with an increasing demand for the securement of energy resources, environmental protection, and the like, and, in particular, as drilling for unconventional resources becomes more widespread, requirements for drilling have become more stringent. Therefore, there has been a demand for degradable materials contained in well treatment fluids such as drilling fluids, fracturing fluids, cementing fluids, temporary plug fluids, and completion fluids to have an optimal composition and shape.
Specifically, there has been a demand for a degradable material which has properties indispensable to well treatment fluids such as, for example, when blended into a fracturing fluid, excellent proppant dispersibility and dispersion stability (due to interactions with the proppant) and an ability to sufficiently secure the pressure of the fracturing fluid, and when blended into a temporary plug fluid, an ability to sufficiently secure the strength of the plug, the degradable material, in particular, having excellent hydrolyzability and biodegradability so as to have the characteristics that the well treatment fluid can be retrieved and disposed of easily and, more preferably, the well treatment fluid disappears in a short period of time without being retrieved or disposed of, even if left behind at the site where the well treatment fluid is applied.